FIR: rewrite lambda return type inference

* `return` should only be added to the last statement if the return
   type is not Unit

 * If there is a `return` without an argument, then the expected return
   type is Unit and the last expression is not a return argument (unless
   it's an incomplete call, in which case it is inferred to return Unit;
   this behavior is questionable, but inherited from K1)

 * There should be a constraint on return arguments even if the expected
   type is Unit, otherwise errors will be missed

 * When the expected type is known, using the call completion results
   writer is pointless (and probably subtly wrong).

^KT-54742 Fixed

compiler/testData/diagnostics/tests/inference/coercionToUnit/afterBareReturn.fir.kt

@@ -34,7 +34,7 @@ fun test4() = <!INFERRED_TYPE_VARIABLE_INTO_EMPTY_INTERSECTION!>run<!> {
 
 fun main() {
     // all ok
-    expectUnit(<!ARGUMENT_TYPE_MISMATCH!>test1()<!>)
+    expectUnit(test1())
     expectUnit(test2())
     expectUnit(test3())
     expectUnit(test4())